The invention is directed to a circuit arrangement for short-circuit and overload protection of a power circuit.
Monolithically integrated digital components can process only relatively small currents at their outputs and in control techniques where digital integrated circuits are, to a large extent, employed for the control of power elements, for example relays, lamps or magnetic control switches, currents up to 1A are required which, for reasons of uniformity in production and thus economical fabrication, should also be integrated monolithically. It is further required in such power circuits, as well as the standard digital circuits involved, that the outputs thereof are not destroyed in the presence of overloads or short circuits. At the mentioned voltages and currents, short circuit power losses up to several multiples of 10 watts may occur, which can instantly destroy the systems and their housings etc. It is therefore necessary that any overload protection provided either cuts off the current or radically reduces it in the presence of a short circuit or overload. If possible such overload protection likewise should be electronically controlled. Also, in the event such an overload ceases, the current should independently reestablish itself without any unduly large time delay.
Further, brief overloads, in such power circuits should be tolerated without responsive action taking place in the protective circuit, if for example, capacitive loads or bulbs with little cold resistance are being operated. If there is no overload the function of the protected circuit should in no way be influenced by the elements of the safety device.
Such a safety device could be achieved thermically, in which case the circuit would have to include a temperature sensor which activates a control circuit, whereby the power loss is reduced in the event of a short circuit or overload, and it is assured that the blocking layer temperatures do not increase over a maximum value at which no damage will occur. However, the construction of such a thermal safety device is quite difficult and high requirements would be necessary with respect to production tolerances. In addition, there is the disadvantage that in such a thermal protective device it would be difficult to maintain the control circuit at a stable level.
In another possibility, when the current exceeds the boundary along a negative resistance characteristic line, it is reduced to a value which is not dangerous. However, tolerance problems will likewise occur. Furthermore an ideal protection in the event of overload is not guaranteed since a sufficient differentiation between a genuine short circuit and the maximum operation cannot be effected. Further, special measures with respect to a delayed function would have to be taken in the operation of bulbs, etc.